Identification of non-immunoglobulin A-Fc-binding forms and low-molecular-weight secreted forms of the group B streptococcal beta antigen.

The beta antigen expressed on the surfaces of certain strains of group B streptococci has been reported to bind to the Fc region of human immunoglobulin A (IgA). In this study, we screened 100 isolates of group B streptococci for expression of both beta antigen and IgA-Fc-binding activity. We identified two isolates which expressed the beta antigen but could not bind human IgA Fc fragments and also observed variability in IgA-Fc-binding activity among other beta-antigen-expressing strains. Novel low-molecular-weight forms of beta antigen were secreted by four beta-antigen surface-negative isolates and included IgA-Fc-binding (Mrs, 55,000 and 53,000) and non-IgA-Fc-binding (Mr, 38,000) molecules. These results suggest that the IgA-Fc-binding site represents a unique domain of the beta antigen. The 55,000- and 53,000-Mr forms of secreted beta antigen were functionally and antigenically representative of the size-heterogeneous (Mr, up to 145,000) beta-antigen molecules expressed by surface-positive strains. The cell surface-localized IgA-Fc-binding molecules could bind only human serum IgA efficiently; however, once solubilized, these molecules could bind both human serum and secretory IgAs.